1. Field of the Invention
The present invention relates to a semiconductor light emitting device, particularly, a semiconductor light emitting device including GaN-based compound semiconductor such as GaN, InGaN, GaAlN and InGaAlN, and a method for adjusting the luminous intensity.
2. Description of the Related Art
As materials for semiconductor light emitting devices such as light emitting diodes (LED) and laser diodes (LD) which emit light in the range of the yellow color to the UV rays, particularly emit blue color light, group III-V compound semiconductors expressed by a general formula of InxGayAlzN (where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6z xe2x89xa61) are known. As these compound semiconductors are of a direct transition type, the light emitting efficiency of these compound semiconductors are generally high. Further, the wavelength of the emitting light can be controlled with the In concentration. Thus, great attention has been paid to the compound semiconductors as a light emitting device material.
Generally, it is very difficult to grow a GaN-based bulk crystal. Therefore, for production of the crystal film, a so-called hetero-epitaxial growth method by which the GaN-based crystal film is grown on a substrate made of a different material is used.
Conventionally, as an underlying substrate for use in the hetero-epitaxial growth method of GaN, a C-plane sapphire substrate has been primarily used. In this case, first, on the C-plane sapphire substrate, a low temperature growth GaN layer and an AlN layer are formed as buffer layers. A crystal film of GaN is grown on these buffer layers. As a result, a GaN layer having a good crystallinity is formed.
However, even if these buffer layers are used, there are many crystal defects in the formed GaN layer due to the difference between the lattice constants of the C-plane sapphire substrate and GaN (the difference between the lattice constants being about 16.1%) Accordingly, crystal films having satisfactory crystallinities have not been obtained. For this reason, it is desired to form a GaN layer having an improved crystallinity. Further, since the C-plane sapphire substrate used as the underlying substrate is very expensive, a method of forming a GaN layer by use of an inexpensive underlying substrate is desired.
Recently, in order to satisfy the above requirements, a technique in which a ZnO film is used as the buffer layer on which the GaN layer is to be formed has been proposed. The c-axially oriented ZnO film has a lattice constant which is relatively equal to that of GaN (the difference between the lattice constants being about 2%). Accordingly, crystal defects caused by the difference between the lattice constants can be inhibited. In addition, as to the ZnO film, the c-axially oriented film can be relatively easily formed substantially on any substrate except some substrates such as the R-plane sapphire and so forth. Thus, when the ZnO film is used as the buffer layer, crystal defects of GaN, caused by the difference between the lattice constants, can be inhibited, and moreover, the underlying substrate on which the GaN layer is formed can be selected from a wide variety of substrates. That is, the GaN layer can be formed by use of a silicon substrate, a glass substrate, and so forth, which are relatively inexpensive, as the underlying substrate.
However, a method of using the c-axially oriented ZnO film as a buffer layer has not been practically used. This is because, heretofore, it has not been possible to form a GaN layer having a sufficient luminous intensity when a c-axially oriented ZnO film has been used as the buffer layer.
In view of the foregoing, the present invention is directed to a GaN-based semiconductor light emitting device having a sufficient luminous intensity and a method for adjusting the luminous intensity of a GaN-based semiconductor light emitting device.
The semiconductor light emitting device comprises a substrate, a ZnO buffer layer formed on the substrate and a GaN-based light emitting layer formed on the ZnO buffer layer, wherein the ZnO buffer layer has an average crystalline grain size of ZnO grains of about 0.45 xcexcm or more or 0.12 xcexcm or less.
The method of adjusting the luminous intensity of a GaN-based semiconductor light emitting device comprising a substrate, a ZnO buffer layer formed on the substrate, and a GaN type semiconductor light emitting layer formed on the ZnO buffer layer, comprises the step of adjusting the intensity of light output of the GaN-based semiconductor light emitting device by changing the average crystalline grain size of the ZnO buffer layer.
According to the present invention, by proper adjustment of the average crystalline grain size of the ZnO constituting the ZnO buffer layer, the GaN layer having a remarkably higher luminous intensity compared to a conventional one can be formed. More concretely, by formation of the ZnO grains of the ZnO buffer layer to have an average crystalline grain size of at least 0.45 xcexcm or of up to 0.12 xcexcm, a GaN layer having a sufficient luminous intensity can be produced. Thus, it is possible to realize a GaN-based light emitting device having a sufficiently great luminous intensity.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.